材料非线性衰减系数的二次谐波测量方法研究

采用有限幅值法测量材料在基波和非线性引起的二次谐波作用下的衰减系数:利用准线性下的KZK方程推导基波和二次谐波的声压分布,并提取波束修正系数;采用短纯音信号进行非线性实验,对检测得到的基波和二次谐波声压进行衍射修正处理,有效抑制衍射对衰减系数测量的不利影响,继而通过线性拟合的方法计算得到更精确的基波和二次谐波的衰减系数。以水为例进行实验,研究了实验测量所得衰减系数的频率依赖关系,结果表明在非线性条件下水的衰减系数与频率间存在较强的线性关系,而线性条件下衰减系数随频率呈现二次方增长的特性则不适用于非线性条件。该研究提出了准确测量非线性声波衰减系数的方法,为更有效地应用非线性超声检测提供理论依据。      

准抛物型多量子阱结构中光学二次谐波

研究在掺杂n-i-p-i的抛物多量子阱结构中,由于非简谐修正项所导致的子带间光学二次非线性响应(对应于x⁽²⁾),计算表明,x⁽²⁾比GaAs体材料高一至二个数量级,而其共振跃迁频率位于中红外区域。 关键词：     

Voigt位型下电介质/反铁磁/电介质结构二次谐波生成非倒易性研究

利用非线性传递矩阵方法研究了Voigt位型下电介质/反铁磁/电介质 结构二次谐波生成的非倒易性. 研究发现外加静磁场反向和电介质层排序翻转均对二次谐波输出产生影响, 出现了二次谐波生成的非倒易性. 二次谐波生成非倒易性频率区域在反铁磁共振区, 此区间正处于THz频段. 随着入射角度的增加, 非倒易性的效果越来越明显. 研究二次谐波生成的非倒易性, 可为反铁磁器件的设计加工提供理论支持.     

等离激元二次谐波的发展与应用

人们借助等离激元结构,可以在亚波长范围内获得、操控局域放大的光场。它与非线性光学结合,衍生出非线性等离激元。等离激元二次谐波因其卓越特性,近年在理论和应用方面均有较大突破。基于流体动力学模型介绍了等离激元结构二次谐波理论,总结了其发展与应用的报道,包括等离激元共振增强二次谐波效率、基于磁场洛伦兹力的二次谐波效应、对二次谐波辐射方向和频率的调控,以及等离激元二次谐波在结构检测、传感、成像等领域的应用。最后,对等离激元二次谐波研究领域可能的发展方向进行了展望。     

非线性左手材料中的二次谐波

基于电磁场理论，推导了无耗非线性左手材料中二次谐波的曼利－诺关系，及相位匹配条件下正向基波与逆向二次谐波的能量转换过程及其空间分布.验证了可将无耗非线性左手材料的入射面作为反射镜，把能量以二次谐波的形式反射.同时分别给出有限厚度介质板中基波和二次谐波的场强分布数值结果，验证了结论的正确性.最后从相位失配角度说明了相位匹配是分析非线性左手材料二次谐波的重要条件.这为研究左手材料的非线性理论奠定了基础.     

非线性声波方程二次谐波高阶近似解及实验验证

非线性声波方程的传统二次谐波二阶近似解不够精确,对测量材料的非线性系数造成较大误差.利用摄动法展开非线性声波方程得到一系列非齐次偏微分方程,根据低阶解的性质拟定高阶特解的形式,通过符号计算求出高次谐波特解,对所有二次谐波成分求和最终获得二次谐波的高阶近似解.在水中开展非线性声学实验验证高阶近似解,结果表明:二次谐波相对...     

二次谐波共焦成像的分辨率

通过与双光子荧光共焦成像过程比较,研究了非线性二次谐波的强度脉冲响应函数并对它进行频域分析,获得了横向、纵向截止频率和通频带.研究结果表明：二次谐波共焦成像具有更高的分辨率.     

组装L-B膜光学二次谐波的实验研究

一种分子形成的多层L-B膜由于相邻层的分子取向相反而使非线性极化抵消,总的X~(2)=0.本文用光学二次谐波方法确定了带有不同极性基团的L-B单分子层的有效非线性系数的大小和符号,对有效非线性系数符号相反的两种单分子层交替组装,使相邻单分子层的非线性极化相互叠加,得到了具有较大二阶非线性系数的组装L-B膜.     

垂直电场下扭转双层石墨烯光学吸收性质的理论研究

层间扭转角度是对石墨烯物理性质宽波段可调谐的一个新参量.本文采用2°<θ<15°扭转角度下的连续近似模型,获得了不同扭转角度双层石墨烯分别在有、无电场下的能带结构,通过电子-光子相互作用跃迁速率,计算模拟了范霍夫奇点附近电子带内跃迁和带间跃迁所引起的光学吸收谱.结果表明,在无外加电场时,带间跃迁吸收峰的位置随着扭转角度的增大而发生从红外到可见光波段的蓝移,且吸收系数增大,带内跃迁的光学吸收系数相对于带间跃迁高出2个数量级;而存在外加电场时,两个范霍夫奇点在波矢空间的位置发生偏移,带间跃迁吸收峰发生分裂,且两个分裂的吸收峰位置随着电场强度的不断增大而反向行进.上述研究结果对石墨烯材料在光电器件方面的应用有一定指导作用.     

次近邻跃迁对石墨烯纳米带输运性质的影响

根据紧束缚模型,利用格林函数的方法,将次近邻跃迁考虑在内,研究了扶手椅型石墨烯纳米带的输运性质.通过数值计算,给出了不同尺寸和不同次近邻跃迁能下系统的能量-电导和电流-电压特征曲线.结果表明,次近邻跃迁对扶手椅型石墨烯纳米带的输运性质有显著的影响.它破坏了电导共振峰关于能量的对称分布,增强了系统的导电性,减小了电子导电偏压阈值,加速了系统输运性能由半导体向导体转变. 次近邻跃迁能和石墨烯纳米带的尺寸越大,这种影响越明显     

Optical second-harmonic generation in ferroelectric chiral smectic polysiloxane

The second-order nonlinear optical properties of ferroelectric liquid crystalline polymer have been studied. Angular phase-matched Second-Harmonic Generation (SHG) is observed in the chiral smectic C (SmC*) phase of siloxane copolymer with chiral mesogen in the side chain. The effective nonlinear optical coefficient d_eff under the phase-matching condition is determined. The intense SHG is observed even in the crystalline phase by cooling down from the SmC* phase under the electric field. The effective coefficient of SHG in the crystalline phase is more than ten times larger than that of the phase-matched SHG in the SmC* phase. The enhancement of SHG in the crystalline phase is observed only in a homeotropically aligned cell and maintained even in the non-biased state for at least several days. The angular dependence of the SHG in the crystalline phase is confirmed.     

Second harmonic generation in composites of ellipsoidal particles with core–shell structure

We study the enhancement of the second-harmonic generation (SHG) coefficient in a random composite consisting of ellipsoidal particles with a core–shell structure in a linear dielectric host. The material making up the ellipsoidal core is assumed to be dielectric, but with a nonlinear susceptibility for SHG. The coating material is assumed to be metallic with a linear susceptibility. The effective SHG coefficient is derived and its expression is related to various local field factors. The numerical calculations of the effective SHG response per unit volume of nonlinear material can be greatly enhanced at certain frequencies. For coated ellipsoidal particles, the core–shell structure and the particle shape allow for tuning of the resonance through the choice of material parameters and/or the ratio of the core to shell volume fraction and the depolarization factor of the particles.     

Efficient second-harmonic generation in nonlinear plasmonic waveguide

We theoretically studied a nonlinear optical process in a hybrid plasmonic waveguide composed of a nonlinear dielectric waveguide and a metal film with a separation of a thin air gap. Owing to the hybridization effect of guided mode and surface plasmon polariton mode, this particular waveguide is able to confine the optical-field in a deep subwavelength scale together with low propagation loss. Based on this, efficient second-harmonic generations (SHG) were revealed at the fundamental wavelength of λ=1.55?μm with good field confinement. The SHG efficiency, as well as the coupling coefficient and mode area, were analyzed and discussed in detail with respect to the structural parameters.     

Excitations of nonlinear local waves described by the sinh-Gordon equation with a variable coefficient

We explore novel excitations in the form of nonlinear local waves, which are described by the sinh-Gordon (SHG) equation with a variable coefficient. With the aid of the self-similarity transformation, we establish the relationship between solutions of the SHG equation with a variable coefficient and those of the standard SHG equation. Then, using the Hirota bilinear method, we obtain a more general bilinear form for the standard SHG equation and find new one- and two-soliton waves whose forms involve two arbitrary self-similarity functions. By an appropriate choice of the smooth self-similarity functions, we determine and display novel localized waves, and discuss their properties. The method used here can be extended to the three- and higher order soliton solutions.     

Imaging theory of nonlinear second harmonic and third harmonic generations in confocal microscopy

ItisreportedrecentlythatnonlinearopticalphenomenonofSHGandTHGhasbeenobservedinmanybiologicaltissues[16].SHGandTHGhavebeenusedtoperformthethree-dimensionalimaginginlivingtissuesandhaveattractedmuchattentionrecently.TherearemanyadvantagesofusingSHGandTHGtoperformthethree-dimensionalimaginginlivingtissues,suchasnoninvasiveandnophotobleaching,inadditiontotheimagingpropertiesofmulti-photonfluorescenceimaging[7—9].Firstly,unlikeinthesingle-andmulti-photonfluorescenceprocesses,onlyvirtualstat…     

粉末倍频系数的模拟研究

根据Kurtz粉末倍频理论,使用光纤光谱仪自行设计搭建一个粉末倍频测试系统,该系统可在基频光波入射方向不变的情况下,通过转动旋转臂在不同方向探测二次谐波,并且具有较高的精确度和灵敏度。使具有位相匹配特性的颗粒尺寸范围扩大为46μm～250μm 。使用波长为1.064μm的基频光,对KIO3晶体粉末的倍频效应进行了测试。测试结果表明,KIO3晶体粉末透射倍频光具有位相匹配特性。这一测试结果与已报道结果相一致。通过数值模拟实验测试曲线,获得了KIO3晶体粉末的倍频系数d2ω的值为9.7×10-12m/V。这一模拟值与已报道的KIO3晶体对该基频光波产生的倍频系数d32的值相吻合。由此可见,该方法切实可行。并且可推广应用到其他晶体材料的非线性特性测试研究。     

Continuous-wave quasi-phase-matched generation of 60 mW at 465 nm by single-pass frequency doubling of a laser diode in backswitch-poled lithium niobate

We report continuous-wave single-pass second-harmonic generation (SHG) in 4-mum -period 0.5-mm-thick backswitch-poled lithium niobate. Pump sources at 920-930 nm include both Ti:sapphire and diode-oscillator-amplifier lasers. SHG of a Ti:sapphire laser at 6.1%/W efficiency, producing 61 mW of power at 460 nm, is demonstrated in 50-mm-long periodically poled lithium niobate samples with a nonlinear coefficient d(eff) approximately 9pm/V, and 60 mW at 465 nm and 2.8%/W efficiency is obtained by SHG of a laser-diode source.     

990 mW average power, 52% efficient, high-repetition-rate picosecond-pulse generation in the blue with BiB3O6

Efficient single-pass second-harmonic generation (SHG) of tunable high-repetition-rate picosecond pulses into the blue is reported in the nonlinear crystal BiB3O6. Using 2.4 ps fundamental pulses from a mode-locked Ti:sapphire laser at 76 MHz and a 10 mm crystal cut for type I (e + e --> o) phase matching in the optical yz plane, second-harmonic average powers as high as 990 mW with excellent stability have been generated at 52% conversion efficiency, and a tunable range of 370-450 nm is demonstrated. From measurements of single-pass SHG in the continuous-wave regime an effective nonlinear coefficient of 3.7 pm/V has been verified for BiB3O6, and direct comparison with beta-BaB2O4 confirms a SHG power enhancement of approximately 23% for the same crystal length. Autocorrelation measurements in a 200 microm crystal of beta-BaB2O4 result in durations of 2.8 ps for the second-harmonic blue pulses.     

晶体球中非相位匹配二次谐波产生及最佳聚焦条件

晶体球法是近年来在国际上发展起来的一种测量非线性系数的新方法。根据聚焦高斯光束二次谐波产生的孔径方程理论,分析了晶体球中的第Ⅰ类非相位匹配二次谐波产生过程,讨论了晶体球中第Ⅰ类非相位匹配二次谐波最佳聚焦参数的选择。报道了LiNbO3晶体球中皮秒激光脉冲抽运的相位匹配二次谐波实验。所得结果与理论预计相符。     

Optical second harmonic generation in asymmetric double triangular quantum wells

The second harmonic generation (SHG) in the asymmetric double triangular quantum wells (DTQWs) is investigated theoretically. The dependence of the SHG coefficient on the right-well width of the DTQWs is studied, and the influence of the applied electric field on SHG coefficient is also taken into account. The analytical expression of the SHG coefficient is analyzed by using the compact density-matrix approach and the iterative method. Finally, the numerical calculations are presented for the typical GaAs/Al_xGa_1−xAs asymmetric DTQWs. The results show that the calculated SHG coefficient in this coupled system can reach the magnitude of 10⁻⁵ m/V, 1–2 orders of magnitude higher than that in step quantum well, and that in double square quantum wells. Moreover, the SHG coefficient is not a monotonic function of the right-well width, but has complex relationship with it. The calculated results also reveal that an applied electric field has a great influence on the SHG coefficient. Applying an appropriate electric field to a DTQW with a wider right well can induce a sharper peak of the SHG coefficient due to the double-resonant enhancement.